There are many different types of data networks, of which Ethernet is perhaps the best known. Some data networks have resource reservation schemes. One such network is HPNA (Home Phoneline Network Alliance) v.3, which is designed to work over existing telephone lines and COAX wiring to create a home/small office network. HPNA v.3 and other such resource reservation networks have a scheduler, described hereinbelow, to guarantee media resources to network devices, to prevent collision between multiple network devices using the same line and to ensure quality of service.
Reference is now made to FIG. 1, which depicts a prior art data network 10, comprising at least two network devices 12 and 14, connected to computers. Network device 12 comprises a modem 16 which may include, among other items, a carrier sensor 20 and a transmitter 24. Network device 14 comprises a modem 18 which may include, among other items, carrier sensor 20, a scheduler 22 and a transmitter 24, which transmits the data being transmitted. Scheduler 22 creates and sends to each device on the network a media access plan (MAP) at the beginning of each cycle. One such MAP, here labeled 40, is shown in FIG. 2, to which reference is now made. FIG. 2 depicts an exemplary prior art MAP. MAP 40 is a detailed schedule of future transmission opportunities (TXOPs) that will be made available to all network devices in the upcoming cycle described by MAP 40 and allocates each opportunity to a particular service. MAP 40 details the start time and length of each and all scheduled TXOPs 44, 48, 50, 54 in the next cycle of transmissions, and assigns each TXOP to a particular network device. For example, TXOP 44 may be the first TXOP and may be assigned to a digital telephony service. TXOP 50 may be the third and it may be assigned to a video stream.
Within data networks, there are generally three types of services, constant bit-rate (CBR) services, variable bit-rate (VBR) services and best effort (BE) services. For CBR services, there is a constant amount of data being transferred at any given time. Exemplary CBR services are digital telephony transmissions. For VBR services, such as a video stream, the amount of data to be transferred varies from transmission to transmission.
The scheduler can easily schedule CBR activity, since the same amount of bandwidth is required for each transmission. VBR is more complicated to schedule, due to its varied nature, and generally the scheduler allocates a fixed amount of bandwidth to be utilized. The amount allocated is typically between peak and average bit-rate requirements for the service. Because VBR flows transmit a variable amount of bits per cycle, the allocated bandwidth may not necessarily be utilized during each cycle.
Best effort services are transmitted during contention periods (CPs), described hereinbelow, during which, the services to be transmitted contend for access to the network. Thus, only those that access the network get transmitted and their level of service is not guaranteed. The data is typically transmitted with a variable number of bits per cycle.
Typically a MAP schedules CBR and VBR flows first, followed by a contention period, during which all devices may transmit BE data on a first-come, first-served basis. In the cycle described by MAP 40, the first CBR TXOP 44 is scheduled to begin after MAP TXOP 42, and a second CBR TXOP 48, may begin as soon as the end time of CBR TXOP 44 occurs. The start time of a first VBR TXOP 50 is scheduled for immediately after the end of CBR TXOP 48. At the end time of VBR TXOP 50 the scheduled start time of a second VBR TXOP 54 may begin. Finally, at the end of the cycle, a CP 60 is scheduled.
After MAP 40 has been sent to all network devices, each device recognizes a particular TXOP that has been assigned to it according to MAP 40, and either utilizes the TXOP or passes on it. Carrier sensor 20 (FIG. 1) within each device may sense if the network medium is available. If it is free to use, the device begins to transmit data.
Reference is now made to FIG. 3, which illustrates an exemplary transmission using MAP 40. First, MAP 40 is sent out by scheduler 22 (FIG. 1). After MAP 40 is received, the transmissions may begin. First, CBR1 64 and CBR2 68 are transmitted during TXOPs 44 and 48, respectively. Immediately after CBR2 68, the first VBR transmissions may start.
The first VBR transmission, VBR1 70 takes place during VBR TXOP 50, but the service does not utilize all the bandwidth allocated. Thus, there is some unused bandwidth 82. After VBR TXOP 50 completes, the next VBR transmission 74 begins during VBR TXOP 54. As with the previous transmission opportunity, VBR2 74 did not utilize all the bandwidth allocated and thus, some of the bandwidth, labeled 84, is wasted.
During contention period (CP) 60, the available bandwidth is made free for BE data transmissions 80, and especially for BE transmissions for which quality of service (QoS) is not guaranteed, and which were not scheduled to be transmitted during the scheduled TXOPs.
As can be seen, the prior art MAP wastes significant resources when the VBR has less to transmit than its allocated bandwidth. Wasting resources also means that any traffic for which QoS parameters are not guaranteed has little opportunity to be transmitted and its packets must be stored until it can be transmitted.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.